Effective control methods for gray mold on chrysanthemums in greenhouses

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Control. The disease is not a problem in greenhouses where the relative humidity is kept at a low level and there is no surface moisture condensation. Chrysanthemums grown in autumn or early winter are affected most frequently and severely because they are kept at low temperatures to prevent premature flowering. High-volume fungicide spraying provides a moderate effect; it should be started even before the disease appears. Regular treatment with zineb yields good results, but it leaves a fungicide coating on the leaves, which impairs the appearance of the flowers. Grey mould of flowers (Botrytis cinerea)

Grey mould is a very common disease. The fungus colonizes flowers that are already affected by another pathogen or damaged by a pest. In conditions of low temperature and high humidity, the grey mould pathogen can also be a primary pathogen, affecting healthy flowers. The first symptoms of grey mould are similar to the damage caused by T. perrexans, but the spots become watery, and the browned tissues are not covered with a white coating. Usually, the outer flowers of the inflorescence are affected first, on which the typical grey fluffy coating of B. cinerea gradually develops. The fungus affects a large number of crops and weed plants; its spores are present in most greenhouses. Under conditions favorable for infection, the symptoms of grey mould appear within a few days, and with the onset of the pathogen's sporulation, the spread of the infection by airborne spores can occur very rapidly.

Control. A slight increase in temperature and ventilation, sufficient for air circulation in the greenhouse, and a reduction in relative humidity are conditions unfavorable for infection. High-volume spraying with iprodione, vinclozolin, or one of the benzimidazoles ensures effective suppression of the pathogen, provided that it is not resistant to these preparations (see p. 131).

Seedlessness, or aspermy (tomato aspermy virus tomato)

Infection with this virus is identified by the deformation of inflorescences, often accompanied by their drooping, twisting, or waviness of the petals, and a general change in color and size. Symptoms of the disease do not appear on the leaves. In plants affected during maturation, symptoms may be weak or completely unnoticeable. Tomatoes are also a host for the aspermy virus (see p. 180).

The only vector for the aspermy pathogen is the green peach aphid. It is a non-persistent virus, for which the aphid requires a very short period of time to accumulate and transmit it. It is poorly transmitted by sap, so the infection rarely spreads when working with affected plants. Cuttings serve as the main source of the disease.

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Control. To sanitize plants affected by aspermy, thermotherapy and meristem culture are used (see p. 77), although these methods are mainly employed by propagation specialists. Growing healthy cuttings from material obtained from the national association for the production of mother plants has led to the almost complete sanitization of most chrysanthemum cultivars. Regular application of insecticides reduces the risk of infection spread by aphids.

In the UK, there are three rarely occurring inflorescence diseases that give typical symptoms. An unidentified Alternaria species causes small chlorotic spots on the petals, usually surrounded by a pink border. These spots grow, darken, and can affect a large part of the flowers. The disease can be suppressed using some agrotechnical and chemical control measures described in the section on grey mould of flowers (see p. 315).

Sometimes downy mildew (pathogen Peronospora sp.) appears on chrysanthemum. Affected inflorescences appear dusted with black powder. Downy mildew sometimes develops on cut chrysanthemums in temporary storage. Maintaining a humidity regime and high-volume spraying are effective against it.

Both in greenhouses and in open field conditions, chrysanthemum is sometimes affected by a Pseudomonas bacterium, which causes inflorescence shedding. The pathogen penetrates the inflorescences as the buds open, upon which black rot develops. Control measures for this infection have not been developed.